Federal Communications Commission of USA (FCC) requires cellular mobile communication systems to be capable of locating mobile stations and explicitly specifies that in the case of assistant measurement of a mobile station, the probability of locating error being less than 50 meters be greater than 67% and the probability of locating error being less than 100 meters be greater than 95%. According to FCC regulations, dedicated technical specifications (25.305 and 25.395) are set down for locating a mobile station in emerging 3G mobile communication systems. Specification 25.395 requires explicitly assistant measurement capability of 3G mobile stations, which is necessary for locating capability. In addition, to ensure measurement performance, specification 25.305 puts forth a unique “Idle Period of Down Link” (IPDL) mode. However, because Non-Line-Of-Sight (NLOS) transmission paths exist ubiquitously in urban areas, there are severe NLOS errors in TDOA (time difference of arrival) measurement in urban areas, thus it is difficult to achieve the locating accuracy required by FCC in urban areas.
In urban areas, except for a few LOS paths, such as paths 103, 104 and 105, NLOS transmission paths exist almost ubiquitously between mobile stations and base stations, such as NLOS paths 106 and 107 in FIG. 1, and they are the primary reason for mobile station locating errors, which may be as high as several hundreds or even several thousands meters, and are difficult to eliminate.
Currently, there are mainly 2 approaches to decrease NLOS errors:
1) Intensive measurement is carried out in the urban area where NLOS exists to obtain a large quantity of empirical data about the distribution of channel multi-paths (amplitude measurement corresponds to the special case of mono-path). Then the position of the mobile station is corrected on the basis of the empirical data. Such an approach that improves the accuracy of locating a mobile station in urban areas through traversal measurement to obtain an empirical database requires vast workload. Moreover, once the urban area environment changes (e.g., when buildings are added or removed), the database will become invalid. To keep the database adapting to the changes of an urban area, usually a large quantity of measurement has to be performed.
2) Several locating reference points (e.g., transponders) are added in the urban area where NLOS exists to correct NLOS error of a mobile station according to the signal characteristics of those reference points. In U.S. Pat. No. 5,926,133, a method for increasing the accuracy for locating a mobile station is disclosed, as shown in FIG. 2. The basic idea of this method is to place some transponders in the area of a specified block 201 as an assistant locating apparatus 202 to improve accuracy for locating a mobile station. The basic principle of improving locating accuracy with the assistant locating apparatus 202 (the transponders) is: the signals sent from the mobile station 204 and the signals transmitted from nearby transponders 202 are received at the base station through similar transmission paths. For each transponder, two locations are obtained: one is the location obtained through measuring (e.g., TDOA) the signals sent from the transponder and then through estimating of the location, and an NLOS error is included in the location; the other is the location measured accurately in advance. Through comparing the two locations, the NLOS error value may be obtained. An NLOS error vector may be created with NLOS errors of the transponders adjacent to the mobile station, and the NLOS error of the mobile station may be corrected on the basis of the NLOS error vector. Cable 203 is used to transfer data and provide power in order to enable the transponder to work. Position 205 of the mobile station is estimated by the assistant locating apparatus under the influence of NLOS. The working process of the method described in the patent is: (1) determining the approximate location of the mobile station 204; (2) starting ambient transponders to determine the approximate locations of those transponders with the signals transmitted by them; (3) constructing an NLOS error vector with the approximate locations of those transponders; (4) correcting the approximate location of the mobile station with the NLOS error vector to obtain the accurate location of the mobile station 204. The advantages of the method are that it doesn't need any assistance from the mobile station, and that the method is suitable for locating mobile stations with up links, and can adapt to the environmental variations of urban areas by adding transponders. However, to ensure the correction accuracy of the NLOS error, transponders have to be configured with enough accuracy, which may result in increased network construction costs. In addition, in 3G communication systems, the location of a mobile station has to be determined through measuring TDOA for down link in the IPDL mode. However, the method and apparatus described in U.S. Pat. No. 5,926,133 cannot be used in the IPDL mode in 3G communication systems.
The object of the present invention is to provide a method and apparatus for effectively increasing the accuracy for locating cellular mobile stations in urban areas, in order to improve the signal transmission environment in NLOS urban areas to keep the probability of NLOS transmission paths in urban areas similar to in suburban areas.